1. Field of the Invention
The present invention relates to surface acoustic wave devices, such as surface acoustic wave resonators or surface acoustic wave filters, and manufacturing methods therefor, and more particularly, relates to an electrode structure of a surface acoustic wave device and a forming method therefor.
2. Description of the Related Art
As has been well known, surface acoustic wave devices are electronic elements including a surface acoustic wave in which mechanical vibration energy is concentrated only in the vicinity of surfaces of a solid material and is then propagated. In addition, the surface acoustic wave devices are each generally composed of a piezoelectric substrate having piezoelectric properties and electrodes, such as interdigital electrodes and/or grating electrodes, disposed on the piezoelectric substrate for processing electrical signals and surface acoustic waves.
In the surface acoustic wave devices described above, as an electrode material, aluminum (Al) having a low electrical resistivity and a low specific gravity or an alloy thereof has been used.
However, since Al has poor stress migration resistance, when a large electrical power is applied thereto, hillocks and/or voids are formed in the electrodes, and short-circuiting or disconnection of the electrodes may occur in some cases, resulting in breakage of the surface acoustic wave device.
In order to solve the problems described above, a method for improving electrical power resistance has been disclosed in Japanese Unexamined Patent Application Publication No. 7-162255 (patent publication 1) in which the crystal orientation is improved by an ion beam sputtering method used as a method for forming electrodes.
In addition, another method for improving electrical power resistance has been proposed in Japanese Unexamined Patent Application Publication No. 3-48511 (patent publication 2) in which an Al crystal is oriented in a predetermined direction by an epitaxial growth method.
Japanese Unexamined Patent Application Publication No. 6-6173 (patent publication 3) has disclosed that electrical power resistance of electrodes can be improved as crystal grain size is decreased.
Furthermore, in xe2x80x9cTechnical Handbook of Surface Acoustic Wave Devicexe2x80x9d edited by the 150th Committee on Technology of Surface Acoustic Wave Device of Japan Society for the Promotion of Science, published by Ohmsha, Ltd., p. 267 (non-patent publication 1), a phenomenon has been disclosed in which the electrical power resistance is improved when copper (Cu) is added to Al.
Patent publication 1: Japanese Unexamined Patent Application Publication No. 7-162255
Patent publication 2: Japanese Unexamined Patent Application Publication No. 3-48511
Patent publication 3: Japanese Unexamined Patent Application Publication No. 6-6173
Non-patent publication 1: xe2x80x9cTechnical Handbook of Surface Acoustic Wave Devicexe2x80x9d edited by the 150th Committee on Technology of Surface Acoustic Wave Device of Japan Society for the Promotion of Science, published by Ohmsha, Ltd., p. 267.
However, by the traditional techniques disclosed in patent publications 1 and 3, recent higher frequency and larger electrical power requirements cannot satisfactorily be fulfilled, and hence, when the techniques described above are used in high-frequency or large electrical power applications, insufficient electrical power resistance becomes a serious problem.
In addition, according to the traditional technique disclosed in patent publication 2, an epitaxial film having superior crystallinity can be actually grown only on a quartz substrate. However, on a substrate composed of a piezoelectric crystal, such as LiTaO3 or LiNbO3, used for filters which have superior piezoelectric properties and are advantageously used in a broad band, it has been difficult to grow an epitaxial film having superior crystallinity by the technique disclosed in patent publication 2, and as a result, the traditional technique described above cannot practically be applied to a surface acoustic wave device including a LiTaO3 or LiNbO3 substrate.
According to the traditional technique disclosed in non-patent publication 1, by adding Cu to Al, the electrical power resistance can actually be improved. However, a level of this improvement has not been satisfactory in practice.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device and a manufacturing method therefore, in which the novel surface acoustic wave device achieves superior electrical power resistance by using an epitaxial Al film having a twin structure for an Al electrode layer which is primarily composed of Al and which constitutes an electrode provided on a piezoelectric substrate formed, for example, of a 64xc2x0 Y-X cut LiNbO3. In this case, it was understood that the epitaxial Al film grows in a particular manner, that is, the (111) plane thereof is oriented with respect to a Z axis of the piezoelectric substrate and has a twin structure which is grown in the (111) plane.
Compared to a single crystal, mechanical strength of an epitaxial film having a twin structure is very high, and as a result, the plastic deformation is unlikely to occur. Accordingly, a significant advantage can be obtained in that electrode breakage of surface acoustic wave devices, which is frequently caused by stress migration, is prevented from occurring.
Through intensive research by the inventors of the present invention regarding the epitaxial Al film having the twin structure described above, it was understood that, in some cases, crystal growth may occur according to a mechanism which is totally different from that in which the epitaxial film grows while the (111) plane of the Al film is oriented with respect to the Z axis as described above. In this case, the Al(111) plane is not oriented along the Z axis of the piezoelectric substrate, and very particular crystal growth occurs in which the Al(111) are oriented in a plurality of directions. The crystal growth described above is observed in particular when a Y-cut piezoelectric single crystal is used as a piezoelectric substrate, and in more particular, when 36xc2x0 to 42xc2x0 Y-cut LiTaO3 substrate is used. Other suitable substrates may also be used.
According to the information thus obtained, a preferred embodiment of the present invention provides a surface acoustic wave device including a piezoelectric substrate made of a piezoelectric single crystal and at least one electrode provided on the piezoelectric substrate, and the at least one electrode has an electrode layer which is an oriented electrode layer formed by epitaxial growth, and the electrode layer is a polycrystalline thin film having a twin structure in which a diffraction pattern observed in an X-ray diffraction pole figure has a plurality of symmetry centers.
The electrode layer described above preferably includes Al as a primary component.
The electrode described above may further include an underlying electrode layer provided between the electrode layer and the piezoelectric substrate for improving the crystallinity of the electrode layer. This underlying electrode layer may include at least one of titanium (Ti) and chromium (Cr) as a primary component.
In addition, the electrode may further include an intermediate electrode layer provided between the Al electrode layer and the underlying electrode layer so as to cause a crystal face present the surface of the underlying layer to be in a cleaner state.
The piezoelectric substrate preferably includes a LiNbO3 or a LiTaO3 single crystal and, more preferably, is a xcex8 rotation Y-cut (xcex8 is between 36xc2x0 and 42xc2x0) LiTaO3 substrate.
Concerning the crystal orientation of the electrode layer provided for the surface acoustic wave device of preferred embodiments of the present invention, in X-ray diffraction in which X-rays are incident on the (200) plane of the crystal constituting the electrode layer, the [111] direction of the crystal is preferably oriented so as to approximately coincide with the center of symmetry spots detected in the X-ray diffraction pole figure.
In addition, the symmetry spots in the X-ray diffraction pole figure preferably have at least two centers, the crystal of the electrode layer may grow in at least two [111] directions, and the [111] directions of the crystal may be oriented so as to approximately coincide with the centers of the symmetry spots detected in the X-ray diffraction pole figure.
In the case described above, the symmetry spots detected in the X-ray diffraction may form three-fold or six-fold symmetry.
According to another preferred embodiment of the present invention, a method for manufacturing a surface acoustic wave device including a piezoelectric substrate formed of a Y-cut piezoelectric single crystal, and at least one electrode formed on the piezoelectric substrate, the electrode including an Al electrode layer primarily composed of Al and an underlying electrode layer provided between the piezoelectric substrate and the Al electrode layer for improving the crystallinity thereof, the Al electrode layer being an oriented film formed by epitaxial growth and being a polycrystalline thin film having a twin structure in which a diffraction pattern observed in an X-ray diffraction pole figure has a plurality of symmetry centers, is provided.
The method for manufacturing the surface acoustic wave device of a preferred embodiment of the present invention includes preparing the piezoelectric substrate formed of the Y-cut piezoelectric single crystal, forming the underlying electrode layer on the piezoelectric substrate, forming the Al electrode layer on the underlying electrode layer, and performing etching treatment for the piezoelectric substrate prior to the step of forming the underlying electrode layer to expose a crystal face on a surface of the piezoelectric substrate so that the Al electrode layer can be formed by epitaxial growth.
The etching step described above is preferably performed using an etchant including at least one selected from the group consisting of phosphoric acid, pyrophosphoric acid, benzoic acid, octanoic acid, hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid, buffered hydrofluoric acid (BHF), and potassium hydrogen sulfate.
In addition, through further detailed investigation regarding the epitaxial Al film having a twin structure, it was understood that crystal growth may occur in some cases according to a mechanism which is totally different from the mechanism described above in which the epitaxial growth proceeds while the (111) plane of the Al film is oriented with respect to the Z axis. The crystal growth described above is observed in particular when a low cut angle substrate, such as a 36xc2x0 Y-cut piezoelectric single crystal, is used as a piezoelectric substrate, and is very particular crystal growth in which the Al(111) planes are oriented in at least two directions which are different from the Z axis of the piezoelectric substrate.
Depending on process conditions in which the underlying electrode layer and/or the Al electrode layer is formed, the crystal growth described above may not be performed in some cases. Through further detailed investigation on the phenomenon described above, it was understood that the Al electrode layer including an epitaxial Al film having a twin structure cannot be obtained unless the underlying electrode layer is formed using Ti by heating to a temperature of 70xc2x0 C. or more, and the Al electrode layer is formed at a relatively low temperature of 50xc2x0 C. or less. The reason for this is that when the Al electrode layer is formed by heating, due to the counter diffusion between Al and Ti, epitaxial growth of Al is inhibited.
A surface acoustic wave device produced by a method according to a preferred embodiment of the present invention includes a piezoelectric substrate, and at least one electrode formed on the piezoelectric substrate, the at least one electrode including an Al electrode layer primarily composed of Al and an underlying electrode layer provided between the piezoelectric substrate and the Al electrode layer for improving the crystallinity thereof. The method according to this preferred embodiment includes the following steps.
That is, the method includes a step of preparing the piezoelectric substrate, a step of forming the underlying electrode layer on the piezoelectric substrate by heating to a temperature of about 70xc2x0 C. or more, and a subsequent step of forming the Al electrode layer at a relatively low temperature of about 50xc2x0 C. or less.
The step of forming the underlying electrode layer by heating is preferably performed at a temperature of about 300xc2x0 C. or less.
In addition, the step of forming the Al electrode layer at a relatively low temperature is preferably performed at a temperature of about 0xc2x0 C. or more.
In preferred embodiments of the present invention, the piezoelectric substrate may include a Y-cut piezoelectric single crystal. In this case, the piezoelectric substrate is preferably a LiNbO3 or a LiTaO3 single crystal and is more preferably a xcex8 rotation Y-cut (xcex8=36xc2x0 to 42xc2x0) LiTaO3 substrate.
In the step of forming the Al electrode layer, the Al electrode layer is preferably grown so as to form an epitaxial film having a twin structure.
In addition, prior to the step of forming the underlying electrode layer, the present invention may further include a step of performing pretreatment for the piezoelectric substrate to expose a crystal face on a surface thereof so that the Al electrode layer can be formed by epitaxial growth. The underlying electrode layer preferably includes at least one of Ti and Cr as a primary component.
The present invention may further include a step of forming an intermediate electrode layer on the underlying electrode layer at a low temperature of about 50xc2x0 C. or less for placing a crystal face present on the underlying electrode layer in a cleaner state, wherein the Al electrode layer is preferably formed on the intermediate electrode layer in the step of forming the Al electrode layer.
The intermediate electrode layer preferably includes at least one of Ti and Cr as a primary component or preferably includes the same material as that for the underlying electrode layer.
In addition, the step of forming the intermediate electrode layer at a low temperature is preferably performed at a temperature of about 0xc2x0 C. or more.
Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.